Vehicle accessory microphone assembly having a windscreen with hydrophobic properties

ABSTRACT

A microphone assembly for a vehicle accessory includes one or more transducers positioned in a microphone housing. The microphone housing has at least one acoustic port to which the transducer is acoustically coupled. The microphone assembly further includes a windscreen sealed across the acoustic port. The windscreen having hydrophobic properties to prevent water from penetrating the microphone housing through the acoustic port.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/724,119, entitled “MICROPHONE ASSEMBLY HAVING A WINDSCREENOF HIGH ACOUSTIC RESISTIVITY AND/OR HYDROPHOBIC MATERIAL,” filed on Nov.28, 2000 by Alan R. Watson et al., which is a continuation-in-part ofU.S. patent application Ser. No. 09/444,176, entitled “VEHICLE ACCESSORYMICROPHONE,” filed on Nov. 19, 1999 by Robert R. Turnbull et al., andwhich is continuation under 35 U.S.C. §120 of International PCTApplication No. PCT/US00/31708, filed on Nov. 17, 2000. U.S. patentapplication Ser. No. 09/724,119 also claims priority under 35 U.S.C.§119(e) on U.S. Provisional Patent Application No. 60/195,509, entitled“VEHICLE REARVIEW MIRROR ASSEMBLY INCORPORATING A COMMUNICATION SYSTEM,”filed on Apr. 6, 2000 by Robert R. Turnbull et al.; on U.S. ProvisionalPatent Application No. 60/216,297, entitled “VEHICLE REARVIEW MIRRORASSEMBLY INCORPORATING A COMMUNICATION SYSTEM,” filed on Jul. 6, 2000 byRobert R. Turnbull et al.; on U.S. Provisional Patent Application No.60/221,307, entitled “AUTOMOTIVE MICROPHONE INTERFACE CIRCUIT,” filed onJul. 28, 2000 by Robert R. Turnbull et al.; and on U.S. ProvisionalPatent Application No. 60/242,465, entitled “VEHICLE REARVIEW MIRRORASSEMBLY INCORPORATING A COMMUNICATION SYSTEM,” filed on Oct. 23, 2000by Robert R. Turnbull et al.

[0002] The disclosures of each of the above-referenced applications areincorporated herein in their entirety.

BACKGROUND OF THE INVENTION

[0003] The present invention pertains to microphones, and moreparticularly to a microphone associated with a vehicle accessory such asa rearview mirror assembly or the housing of a rear vision displaydevice.

[0004] It has long been desired to provide improved microphoneperformance in devices such as communication devices and voicerecognition devices that operate under a variety of different ambientnoise conditions. Communication devices supporting hands-free operationpermit the user to communicate through a microphone of a device that isnot held by the user. Because of the distance between the user and themicrophone, these microphones often detect undesirable noise in additionto the user's speech. The noise is difficult to attenuate. Aparticularly challenging hands-free application where dynamicallyvarying ambient noise is present is a hands-free communication systemfor a vehicle. For example, bi-directional communication systems such astwo-way radios, cellular telephones, satellite phones, and the like, areused in vehicles, such as automobiles, trains, airplanes and boats. Fora variety of reasons, it is preferable for the communication devices ofthese systems to operate hands-free, such that the user need not holdthe device while talking, even in the presence of high ambient noiselevels subject to wide dynamic fluctuations.

[0005] Bi-directional communication systems include an audio speaker anda microphone. In order to improve hands-free performance in a vehiclecommunication system, a microphone is typically mounted near thedriver's head. For example, a microphone is commonly attached to thevehicle visor or headliner using a fastener such as a clip, adhesive,hook and loop fastening tape (such as VELCRO® brand fastener), or thelike. The audio speaker associated with the communication system ispreferably positioned remote from the microphone to assist in minimizingfeedback from the audio speaker to the microphone. It is common, forexample, for the audio speaker to be located in a vehicle adaptor, suchas a hang-up cup or a cigarette lighter plug used to provide energizingpower from the vehicle electrical system to the communication device.Thus, although the communication system designer knows the position ofthe audio speaker in advance, the position of the microphone is unknownas the user can position the microphone where they choose. The positionof the microphone relative to the person speaking will determine thelevel of the speech signal output by the microphone and may affect thesignal-to-noise ratio. The position of the microphone relative to theaudio speaker will impact on feedback between the speaker andmicrophone. Accordingly, the performance of the audio system is subjectto the user's installation of the microphone. Additionally, themicrophone will typically include a wire, which if it is mounted to thesurface of the vehicle interior, will not be aesthetically pleasing.Alternatively, if the wire is to be mounted behind the interior lining,the vehicle interior must be disassembled and then reattached so thatthe wire can be hidden, which may result in parts that rattle loudly orhang loosely from the vehicle frame.

[0006] One potential solution to avoid these difficulties is disclosedin U.S. Pat. No. 4,930,742, entitled “REARVIEW MIRROR AND ACCESSORYMOUNT FOR VEHICLES”, issued to Schofield et al. on Jun. 5, 1990, whichuses a microphone in a mirror mounting support. Although locating themicrophone in the mirror support provides the system designer with amicrophone location that is known in advance, and avoids the problemsassociated with mounting the microphone after the vehicle ismanufactured, there are a number of disadvantages to such anarrangement. Because the mirror is positioned between the microphone andthe person speaking into the microphone, a direct unobstructed path fromthe user to the microphone is precluded. Additionally, the location ofthe microphone on the windshield detrimentally impacts on microphonedesign flexibility and overall noise performance of the microphone.

[0007] U.S. Pat. Nos. 5,940,503, 6,026,162, 5,566,224, 5,878,353, and D402,905 disclose rearview mirror assemblies with a microphone mounted inthe bezel of the mirror. None of these patents, however, discloses theuse of acoustic ports facing multiple directions nor do they disclosemicrophone assemblies utilizing more than one microphone transducer. Thedisclosed microphone assemblies do not incorporate sufficient noisesuppression components to provide output signals with relatively highsignal-to-noise ratios, and do not provide a microphone having adirectional sensitivity pattern or a main lobe directed forward of thehousing and attenuating signals originating from the sides of thehousing.

[0008] It is highly desirable to provide voice recognition systems inassociation with vehicle communication systems, and most preferably,such a system would enable hands-free operation. Hands-free operation ofa device used in a voice recognition system is a particularlychallenging application for microphones, as the accuracy of a voicerecognition system is dependent upon the quality of the electricalsignal representing the user's speech. Conventional hands-freemicrophones are not able to provide the consistency and predictabilityof microphone performance needed for such an application in a controlledenvironment such as an office, let alone in an uncontrolled environmentsuch as an automobile.

[0009] Accordingly, there is a need for a microphone for a vehicleproviding improved hands-free performance and preferably enabling voicerecognition operation.

[0010] Historically, automotive microphones have utilized a two wireinterface to provide an audio signal from the microphone assembly to anelectronic assembly (e.g., an amplifier stage). This two wire interfacehas also provided a power source to the microphone assembly and awetting current through the interface such that reliable continuity wasmaintained between the microphone and the electronic assembly (see FIG.35 and the description below).

[0011] Digital signal processors (DSPs) or other more advanced circuitrythat may be used within a microphone assembly require more power thancan normally be delivered through a standard two wire interface. Assuch, microphone assemblies incorporating DSPs may also require anauxiliary power source to be incorporated within the microphoneassembly. However, implementing an auxiliary power source within amicrophone assembly can introduce ground loops. Further, whennon-precious metal contacts are used in a connector of a microphoneinterface, the contacts of the interface are prone to oxidation, whicheventually leads to a continuity problem between the microphone assemblyand the electronic assembly.

[0012] Thus, what is needed is a microphone interface for automotivemicrophone assemblies that include a power source that provides reliablecontinuity.

SUMMARY OF THE INVENTION

[0013] An aspect of the present invention is to provide a vehicleaccessory having superior speech separation in the presence of noise.Another aspect of the present invention is to provide a vehicleaccessory with enhanced performance for use in hands-free devices,including highly sensitive applications such as voice recognition for avehicle telecommunication system.

[0014] To achieve these and other aspects and advantages, the vehicleaccessory of the present invention comprises a housing; at least onetransducer functioning as a microphone, the at least one transducerpositioned in the housing; and a circuit coupled to the transducer foroutputting an electrical signal such that the microphone has a main lobedirected forward of the housing and attenuating signals originating fromthe sides of the housing.

[0015] According to another embodiment of the present invention, arearview mirror assembly is provided for achieving the above and otheraspects and advantages, which comprises a rearview mirror housing; amirror positioned in the rearview mirror housing; a microphone housingmounted on the rearview mirror housing, the microphone housing having atleast one front port and at least one rear port; and at least onetransducer positioned in the microphone housing, the at least onetransducer including openings ported to the at least one front port andat the at least one rear port such that the microphone has a directionalsensitivity pattern.

[0016] Another embodiment of the inventive vehicle accessory comprisesat least one first transducer; at least one second transducer, whereinthe first and second transducers are positioned in spaced relation; anda circuit coupled to the first and second transducers for combining theoutput signal of the first and second transducers to produce an audiosignal with a reduced noise component.

[0017] The vehicle accessory may include a housing in which thetransducers are positioned. Additionally, the housing may be mounted ona vehicle rearview mirror assembly. According to one embodiment of thepresent invention, the housing includes a deflector disposed proximatethe transducers to deflect airflow away from the transducers. Thedeflector or other part of the housing may optionally include a fineturbulence generator disposed on at least a portion of its surface tocreate fine turbulence in air flowing around the deflector. According toyet another embodiment, the housing has an acoustic port, and awindscreen sealed across the acoustic port. The windscreen may havehydrophobic properties to prevent water from penetrating the housingthrough the acoustic port. The windscreen preferably has an acousticresistivity of at least about 1 acoustic Ω/cm².

[0018] According to another embodiment, the vehicle accessory mayinclude: a first housing having at least one acoustic port, wherein thefirst transducer is disposed in the first housing and acousticallycoupled to the acoustic port of the first housing; a first windscreendisposed across the acoustic port of the first housing; a second housinghaving at least one acoustic port, wherein the second transducer isdisposed in the second housing and acoustically coupled to the acousticport of the second housing; and a second windscreen disposed across theacoustic port of the second housing. With this arrangement, the firstand second windscreens may have different acoustic resistivity, and theacoustic ports of the first and second housings may be configureddifferently, to compensate for differences, or create differences, inthe polar patterns of the transducers.

[0019] In one embodiment, the vehicle accessory further includes acircuit board having a hole sized to receive at least a portion of thefirst and second transducers, wherein the transducers are mounted withinthe hole in the circuit board such that a portion of the transducersextends below a bottom surface of the circuit board.

[0020] According to one embodiment of the invention, the firsttransducer is positioned in front of the second transducer to provide asecond order microphone. According to another embodiment of theinvention, the vehicle accessory may include a mechanical structuredisposed between the transducers to increase the acoustic path lengthbetween the transducers. The circuit may be configured to subtract thesignal from the at least one first transducer from the signal from theat least one second transducer.

[0021] In one embodiment, the vehicle accessory further includes a highpass filter for filtering out low frequency components of audio signalgenerated by the second transducer, and the combining circuit subtractsat least a portion of one audio signal from the other to generate anaudio output signal.

[0022] According to another embodiment, the first transducer receives anaudio signal including a speech signal and noise, and generates a firstelectrical signal representative of the received audio signal, while thesecond transducer receives an audio signal including noise, andgenerates a second electrical signal representative of the receivedaudio signal. The vehicle accessory may further include a speechdetector coupled to the first and second transducers for detecting thepresence of speech; a variable gain amplifier coupled to the secondtransducer for selectively adjusting the gain of the second electricalsignal in response to a gain adjustment signal; and a control circuitcoupled to the first and second transducers, the speech detector, andthe variable gain amplifier for generating the gain adjustment signal asa function of the levels of the first and second electrical signalsreceived from the transducers and in response to a detection of speechby the speech detector.

[0023] Another aspect of the present invention is to provide an audiosystem having superior speech separation in the presence of noise.Another aspect of the present invention is to provide an audio systemwith enhanced performance for use in hands-free devices, includinghighly sensitive applications such as voice recognition for atelecommunication system.

[0024] To achieve these and other aspects and advantages, the audiosystem of the present invention comprises a microphone for receiving anaudio signal including a speech signal and noise, and for generating anelectrical signal representative of the received audio signal, and afilter coupled to the microphone for receiving the electrical signalgenerated by the microphone and filtering the electrical signal tosignificantly reduce the noise and produce a filtered electrical signalincluding the received speech signal. The filter includes a plurality ofnarrow passbands at frequencies spaced from each other by apredetermined frequency corresponding to a fundamental frequency in thespeech signal. The filter thereby blocks frequency components of thereceived audio signal that lie between the plurality of narrowpassbands.

[0025] According to another embodiment of the present invention, anadaptive filter is provided for removing noise from an audio signalincluding a speech component signal. The adaptive filter of the presentinvention comprises a digital signal processor configured to: convert areceived analog signal into a digitized audio signal; identify afundamental frequency and harmonics in the speech component of thedigitized audio signal; provide an inverse comb filter; and pass thedigitized audio signal through the inverse comb filter to filter outfrequency components of the digitized audio signal that do notcorrespond to the identified harmonic frequencies. The digital signalprocessor may further be configured to convert the filtered digitizedaudio signal into an analog signal for output from the digital signalprocessor. The digital signal processor identifies the fundamentalfrequency by (a) performing a fast Fourier transform on the receivedaudio signal, (b) identifying frequency components in the fast Fouriertransform that have amplitudes exceeding a predetermined threshold, and(c) identifying the fundamental frequency as the difference in frequencyof those frequency components having an amplitude above thepredetermined threshold.

[0026] The present invention is also directed to a technique forproviding reliable continuity through a two wire microphone interfacethat removably couples a microphone to an electronic assembly. Themicrophone includes a power source and the two wire microphoneinterface, which includes two contacts that provide an audio signal tothe electronic assembly. A continuous direct current is provided throughthe two contacts such that a low impedance path is maintained betweenthe microphone and the electronic assembly.

[0027] These and other features, advantages and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims, andappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The subject matter that is regarded as the invention isparticularly pointed out and distinctly claimed in the claim portionthat concludes the specification. The invention, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description taken in conjunction with the accompanyingdrawings, where like numerals represent like components, and in which:

[0029]FIG. 1 is a top plan view illustrating a vehicle with a portion ofthe roof cut away;

[0030]FIG. 2 is a front, bottom and left side perspective viewillustrating a rearview mirror assembly and fragmentary mirror supportused in the vehicle of FIG. 1;

[0031]FIG. 3 is a top exploded view illustrating a microphone assemblyused in the mirror according to FIG. 2;

[0032]FIG. 4 is a bottom plan view illustrating the microphone assemblyaccording to FIG. 2;

[0033]FIG. 5 is a bottom plan view illustrating a transducer mount inthe microphone assembly according to FIG. 3;

[0034]FIG. 6 is cross-sectional view taken along plane 6-6 in FIG. 4illustrating the microphone assembly according to FIG. 3;

[0035]FIG. 7 is a top plan view illustrating the microphone assemblyaccording to FIG. 5 with the circuit board removed to view show thetransducers in transducer mount;

[0036]FIG. 8 is a circuit schematic partially in block diagram formillustrating a circuit employed with the microphone assembly of FIGS.3-7;

[0037]FIG. 9 is a top plan view schematic representation illustratingthe sound channel for the transducers of the microphone assemblyaccording to FIGS. 1-7;

[0038]FIG. 10 is a top plan view schematic representation illustratingthe sound channel for an alternate transducer arrangement for themicrophone assembly;

[0039]FIG. 11 is a top plan view schematic representation illustratingthe sound channel for another alternate transducer arrangement for themicrophone assembly;

[0040]FIG. 12 is a circuit schematic partially in block diagram formillustrating a circuit for use with the microphone according to claim11;

[0041]FIG. 13 is a circuit schematic partially in block diagram formillustrating an auto-calibration circuit for use with the microphoneassembly;

[0042]FIG. 14 is a flow chart representing operation of the controllerof FIG. 12;

[0043]FIG. 15 is a cross-sectional view of the microphone according toFIG. 10 taken along the longitudinal axis of the microphone;

[0044]FIG. 16 is a perspective view of a microphone assembly constructedin accordance with another embodiment of the present invention;

[0045]FIG. 17 is an exploded perspective view of a microphone assemblyshown in FIG. 16;

[0046]FIG. 18 is a front isometric view of an embodiment of a rearviewmirror assembly constructed in accordance with another embodiment of thepresent invention;

[0047]FIG. 19 is a rear isometric view of an embodiment of a rearviewmirror assembly shown in FIG. 18;

[0048]FIG. 20 is a side elevation of the rearview mirror assembly shownin FIGS. 18 and 19;

[0049]FIG. 21 is an exploded perspective view of a microphone assemblyconstructed in accordance with another embodiment of the presentinvention;

[0050] FIGS. 22A-22D are plots of polar patterns at differentfrequencies as obtained from a microphone assembly constructed inaccordance with the present invention with a cover over the transducers;

[0051] FIGS. 23A-23D are plots of polar patterns at differentfrequencies as obtained from a microphone assembly constructed inaccordance with the present invention without a cover over thetransducers;

[0052]FIG. 24 is a side elevational view of a portion of a rearviewmirror assembly having a deflector, a fine turbulence generator and amicrophone assembly according to another embodiment of the presentinvention;

[0053]FIG. 25 is a top view of the portion of the rearview mirrorassembly having the deflector, the fine turbulence generator and themicrophone assembly that are shown in FIG. 24;

[0054]FIG. 26 is a rear view of the portion of the rearview mirrorassembly having the deflector, the fine turbulence generator and themicrophone assembly that are shown in FIGS. 24 and 25;

[0055]FIG. 27 is an electrical circuit diagram in block form showing anembodiment of a microphone processing circuit of the present invention;

[0056]FIG. 28A is an electrical circuit diagram in schematic formshowing an examplary high pass filter that may be used in the circuitshown in FIG. 27;

[0057]FIG. 28B is an electrical circuit diagram in schematic formshowing an examplary all-pass phase shifter that may be used in thecircuit shown in FIG. 27;

[0058]FIG. 28C is an electrical circuit diagram in schematic formshowing an examplary summing circuit that may be used in the circuitshown in FIG. 27;

[0059]FIG. 28D is an electrical circuit diagram in schematic formshowing an examplary three-pole high pass filter that may be used in thecircuit shown in FIG. 27;

[0060]FIG. 28E is an electrical circuit diagram in schematic formshowing an examplary buffer circuit that may be used in the circuitshown in FIG. 27;

[0061]FIG. 29A is a plot of three frequency response curves of a secondorder microphone assembly with sound originating from three differentdirections;

[0062]FIG. 29B is a plot of a frequency response curve of the secondorder microphone processing circuit shown in FIG. 27 but without theall-pass phase shifter;

[0063]FIG. 29C is a plot of four frequency response curves of the secondorder microphone processing circuit shown in FIG. 27 with soundoriginating from four different directions;

[0064]FIG. 30 is block diagram illustrating a microphone systemconstructed in accordance with the present invention;

[0065]FIG. 31 is a process diagram for the digital signal processorshown in FIG. 30 according to a first embodiment;

[0066]FIG. 32 is an examplary plot of a FFT of an audio signal receivedfrom a typical transducer while receiving both noise and a user'sspeech;

[0067]FIG. 33 is a graph of an ideal inverted comb filter for filteringthe audio signal whose FFT is illustrated in FIG. 32;

[0068]FIG. 34 is a process diagram for the digital signal processorshown in FIG. 30 according to a second embodiment;

[0069]FIG. 35 is a simplified electrical schematic of a prior artmicrophone assembly coupled to an electronic assembly;

[0070]FIG. 36 is a simplified electrical schematic of a microphoneassembly coupled to an electronic assembly through a microphoneinterface, according to an embodiment of the present invention;

[0071]FIG. 37 is a simplified electrical schematic of a microphoneassembly coupled to an electronic assembly through a microphoneinterface, according to another embodiment of the present invention; and

[0072]FIG. 38 is a simplified electrical schematic of a microphoneassembly coupled to an electronic assembly through a microphoneinterface, according to yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0073] The microphone assemblies of the present invention are associatedwith an interior rearview mirror and have superior performance even inthe presence of noise. The microphone assemblies enhance the performanceof hands-free devices with which they are associated, including highlysensitive applications such as voice recognition for a telecommunicationsystem, by improving the signal-to-noise ratio of the microphoneassembly output. The microphone assemblies eliminate mechanicallyinduced noise and provide the designer with significant freedom withrespect to selection of the microphone assembly's sensitivity, frequencyresponse and polar pattern. Additionally, circuitry can be provided forthe transducer to generate an audio signal from the transducer outputthat has a high signal-to-noise ratio.

[0074] A vehicle 100 (FIG. 1) includes an interior rearview mirrorassembly 101 by which the vehicle operator 103 (illustrated in phantom)can view a portion of the road behind the vehicle 100 without having toturn around. The rearview mirror assembly 101 is mounted to the vehiclewindshield 105, or the vehicle's headliner, via a mirror mountingsupport 104, in a conventional manner that facilitates electricalconnection of the rearview mirror to the vehicle's electrical system andpermits driver adjustment of the mirror-viewing angle.

[0075] The rearview mirror assembly 101 is enlarged in FIG. 2. Themirror assembly 101 includes an elongated housing 206 pivotably carriedon mirror support 104. The mirror 202 may be any conventional interiorrearview mirror, such as a prismatic mirror of the type used with amirror housing manually adjustable for daytime and nighttime operation,or a multiple element mirror effecting automatic reflectivityadjustment, such as an electrooptic or electrochromic mirror. Theelongated housing 206 may be of any conventional manufacture such asintegrally molded plastic.

[0076] The rearview mirror assembly 101 further includes a microphoneassembly 208 that is preferably mounted to the housing 206 at a locationvisible to the vehicle driver 103 or at a position which is direct lineof sight between the speaker's mouth and the microphone. It isadvantageous for the microphone assembly 208 to be positioned on themirror housing 206 as the mirror assembly is movably carried on thesupport 104. The driver 103 (FIG. 1) will typically adjust the positionof the mirror 202 and housing 206 to reflect images visible through therear window 109 of the vehicle 100. When making such an adjustment forviewing angle, the driver 103 adjusts the mirror 202 toward their eyesby moving housing 206, which will simultaneously direct the front ofmicrophone assembly 208 toward the driver. However, the microphoneassembly could be mounted in other vehicle accessories, such as a visor,an overhead console, a vehicle trim component such as a headliner or anA-pillar cover, a center console, or the like.

[0077] A first embodiment of the microphone assembly 208 will now bedescribed in greater detail with respect to FIGS. 3-7. The microphoneassembly includes a microphone housing 300, a transducer mount 302, afirst transducer 304, a second transducer 306, and a circuit board 308.The microphone housing 300 (FIGS. 3 and 4) is generally cylindrical,having a round foot print and a low profile, although the housing couldhave a generally square foot print, an elongated elliptical orrectangular foot print, or any other shape desired by the microphonedesigner. The microphone housing 300 includes front ports 312 that facethe driver 103 and rear ports 314 that face away from the driver 103.The ports 312 and 314 provide a sound passage through the microphonehousing. The ports 312, 314 can have any suitable opening shape or size.The housing also includes posts 316, 317 used to hold the microphoneassembly 208 together, as described in greater detail herein below. Arail 318 on the inside surface of housing 300 is shaped to receive aportion of mount 302. When received in the rail, mount 302 is positionedwith the transducer 304 and 306 sound channels properly aligned with theports 312, 314. The housing also includes mounting tabs 320 forinsertion into openings (not shown) in the lower surface of housing 206.For example, the tabs can be generally L-shaped in profile for insertioninto the housing 300. After tabs 320 are inserted into housing 206, themicrophone housing 300 is locked to the mirror housing 206 by rotatingthe microphone to a locked position, thereby securing the microphoneassembly 208 on the housing assembly 101. Alternately, the tabs 320 canbe elongate snap connectors that slide into an opening (not shown) inthe bottom surface of the mirror housing and snap into engagement withthe inside surface of the mirror housing 206 after full insertion. Themicrophone housing 300 can be integrally molded plastic, stamped metal,or of any other suitable manufacture.

[0078] The transducer mount 302 is configured such that it is pressedinto the housing 300 and is slightly compressed between circuit board308 and housing 300. The transducer mount provides acoustic seals forthe transducers 304 and 306, and with the circuit board 308 and housing300, defines acoustic channels, or sound passages, to the front and rearfaces of the transducers 304, 306, as described in greater detail below.The mount 302 includes webs 324 between walls 332 and webs 325 betweenwalls 333 that extend outwardly from the core of mount 302 to providesound passages, and also help to position mount 302 in the housing 300.Projections 326, 327 are located on opposite ends of mount 302 to helpposition mount 302 in housing 300. Openings 328, 329 are provided in thewebbing 324, 325 of mount 302 for passage of posts 316, 317. Cylindricalwells 330, 331 are provided in the core of transducer mount 302 forreceipt of transducers 304, 306, respectively. Each of the wells 330,331 includes a terminating wall 501 (FIG. 5) against which the frontfaces 500 of the transducers 304, 306 sit. The terminating walls 501each include a channel 506, 508 that extends radially outward from thecenter of the well, which is the location of the front transduceraperture. The mount 302 can be of any suitable manufacture, such as amolded elastomer. In particular the mount 302 is resilient andnon-conductive, and provides acoustic isolation. For example, thetransducer mount 302 can be manufactured of urethane commerciallyavailable from Mobay.

[0079] The transducers 304 and 306 are preferably substantiallyidentical. The transducers include a front aperture 502 which passessound to the front surface of a transducer diaphragm and openings 337(FIG. 3) in the back face that port sound to the back surface of thetransducer diaphragm. The transducers include electrical leads 336 onthe back face thereof for electrical connection to the conductive layerof circuit board 208. The transducers 304 and 306 can be any suitable,conventional transducers, such as electret, piezoelectric, or condensertransducers. The transducers may be, for example, electret transducerssuch as those commercially available from Matsushita of America (doingbusiness as Panasonic), and may advantageously be unidirectionaltransducers. If electret transducers are employed, the transducers canbe suitably conditioned to better maintain transducer performance overthe life of the microphone assembly 208. For example, the diaphragms ofthe transducers 304, 306 can be baked prior to assembly into thetransducers.

[0080] The circuit board 308 has a conductive layer, on surface 334,etched and electrically connected to the transducer leads 336 oftransducers 304, 306. The microphone leads 340 are connected to thetransducer leads 336 by a circuit 800 (FIG. 8) mounted to the conductivelayer of circuit board 308. Although circuit 800 can be mounted on thecircuit board 308 in the microphone housing, it will be recognized thatthe circuit 800 can alternatively be mounted on a printed circuit boardin the mirror housing 206, and further that in the case of anelectrooptic mirror, such as an electrochromic mirror, the circuit 800can be mounted on a common circuit board with the mirror electricalcomponents, or the circuit 800 and the mirror electrical components canbe mounted on separate circuit boards within the housing 206. Theelectrical connection of the microphone leads 340, the transducer leads336, and the components of circuit 800, are preferably by electricaltraces in the conductive layer of the circuit board, formed byconventional means such as etching, and vias extending through thedielectric substrate of the printed circuit board. The circuit boardincludes holes 350 and 352 for receipt of posts 316 and 317 onmicrophone housing 300. The posts 316, 317 are heat staked to thecircuit board substrate after the posts are inserted through holes 350and 352 to secure the connection of the circuit board to the housing 300and insure that the microphone assembly provides acoustically isolatedsound channels between the transducers 304, 306 and the ports 312, 314,as described in greater detail herein below.

[0081] To assemble the microphone assembly 208, the transducers 306 and308 are mounted on the circuit board 308 by conventional means, such asby soldering transducer leads 336 to the conductive layer 334 of circuitboard 308. It is envisioned that the transducer leads can alternativelybe elongated posts that extend through vias in the printed circuitboard, that the surface 360 can be a conductive layer, and that thecomponents of circuit 800 can be located on surface 360 of the printedcircuit board, connected between the transducer leads 336 and themicrophone leads 340. Regardless of how the transducers 304 and 306 aremounted on the circuit board 308, the circuit board mounted transducersare pressed into the cylindrical wells 330, 331 in the mount 302. Whenfully inserted in the wells, the front faces 500 (FIG. 5) of thetransducers 304, 306, are positioned against the terminating wall 501 ofthe wells 330, 331. The wall 501 of each of the wells 330, 331 includesa channel 506, 508 aligned with the openings 502 in the front face ofthe transducers 304, 306.

[0082] The partial assembly comprising mount 302, transducers 304, 306and circuit board 308, is pressed into the housing 300. FIG. 7illustrates the microphone assembly 208 with the printed circuit board308 removed. The back surfaces of the transducers 304, 306, havingmultiple openings 337 and transducer leads 336, are visible from theopen end of the cylindrical wells 330, 331. When the transducers 304,306 are fully inserted in the well, such that the front face 500 of thetransducers are juxtaposed with the wall 501 terminating the well, achamber is formed between the back surface of each of the transducers304, 306 and the circuit board 308, as best shown in FIG. 6. A wall ofthe mount circumscribes the periphery of the transducer 306, 307, and ashort channel 371, 373 extends from the well 330, 331 to the aperture370, 372. The circumscribing wall provides an acoustic seal with thecircuit board 308. Apertures 370, 372 connect the chamber, between eachof the transducers 304, 306 and the circuit board 308, with the channels510, 512, respectively. The chamber behind each of the transducersprovides a sound passage from the back openings 337 of the transducersthrough channels 371, 373, 510, and 512 and ports 312, 314. When themount 302 is fully inserted in the housing 300, the sound passagesextending from the front face of each of the transducers to ports 312and 314 are defined by the housing 300 and the mount 302. The soundpassages extending from the back face of each of the transducers toports 312 and 314 are defined by the housing 300, mount 302 and circuitboard 308.

[0083] In particular the front opening 502 of transducer 306 isconnected to the front ports 312 of the microphone housing 300 via thesound passage 506 as best shown in FIG. 6. The rear face openings 337 ofthe transducer 306 is acoustically coupled to the rear ports 314 viasound channel 373, aperture 372 and channel 510. Transducer 304 iscoupled to the front ports 312 and the rear ports 314 in the samemanner, but in the opposite phase. In particular, the front face oftransducer 304 is acoustically coupled to the rear ports 314 viaacoustic channel 508 (FIG. 5). The rear face openings 337 of thetransducer 304 are acoustically coupled to the front ports 312 viachannel 371, aperture 370, and channel 512. Signals originating from thefront of the microphone assembly, which is the surface of the microphoneassembly facing the driver, enter the front of transducer 306 and theback of transducer 304, whereas sound originating from the rear of themicrophone assembly enter the front face of transducer 304 and the backface of transducer 306. Omni-directional sounds will be detected equallyby the transducers, at opposite phases.

[0084] As illustrated in FIG. 6, the center axes C of the transducers304, 306 are oriented at an angle of 90 degrees with respect to thelongitudinal axes L_(B) and L_(F) of the channels 506, 508, 510, 512.Thus, the acoustic outputs from the two transducers lie on a common axisfacing in opposite directions and perpendicular to the center axis C ofthe transducers.

[0085] The transducers 304 and 306 are electrically coupled to anoperational amplifier 802 (FIG. 8) of circuit 800. In particular,transducer 306 is coupled to the inverting input of the operationalamplifier 802 and transducer 304 is coupled to the non-inverting inputof the operational amplifier. Resistor R8, connected between thetransducer 306 and the inverting input of the operational amplifier 802,is preferably a potentiometer to permit manual balancing of thetransducers. Alternatively, the resistor R12 connected betweentransducer 304 and the non-inverting input of the operational amplifier,or both resistors R10 and R12, can be implemented by potentiometers. Itis also envisioned that a variable gain amplifier with an associatedmanually adjustable potentiometer can be inserted in one or both of thepaths between transducers 304, 306 and operational amplifier 802. Theoperational amplifier may be implemented using any suitable operationalamplifier, such as the TLC271 operational amplifier available from TexasInstruments. The manually adjustable potentiometer R8 is provided forvarying the gain of the transducer path to permit adjustment of thesignal level from transducer 306 such that both transducer 304, 306paths produce the same signal gain (i.e., the signal gain through bothtransducers is equal). By providing identical gain through bothtransducers, omni-directional noise detected by both transducers will becompletely cancelled at the output of the operational amplifier 802.Acoustic signals generated by the vehicle driver, such as the driver'sspeech, will be input to the front of transducer 306 and the back oftransducer 304, such that the speech will be present in the audio signalat the output of operational amplifier 302. Sound from the sides of themicrophone assembly will be cancelled by the transducers 304, 306 andthe operational amplifier 802. The most intense noise in a vehicle tendsto originate from the sides and/or of the vehicle. The microphoneassembly 208 mounted on the rearview mirror 206, including amplifier802, will significantly reduce noise as the bi-directional microphoneassembly is not responsive to noise originating from the sides of thevehicle when mounted in the mirror assembly 101 which is generallyaligned with the longitudinal axis of the vehicle. Furthermore,mechanical noise, such as that originating in the rearview mirrorassembly 101, will be detected by both transducers 304, 306 equally, andthus will be cancelled out by the operational amplifier 802.

[0086] The output of the operational amplifier 802 is input to a 3-polehigh pass filter and unity gain follower 804, having a cut-off atapproximately 100-300 Hz, and preferably at 150 Hz. The filter removesnoise below the voice frequency. Terminals 340 are coupled to thevehicle's electrical circuitry, which may for example include voicerecognition circuitry, a cellular transceiver, a two-way radio, or anyother control circuitry. The transistors Q1 and Q2 can be implementedusing any suitable commercially available transistor elements, such asFFB2227 commercially available from Fairchild Semiconductor.

[0087] In summary, the bi-directional microphone assembly 208 is veryresponsive to voice signals from the driver 103 located in front of themirror assembly 101, as signals from the front of the mirror will sum inoperational amplifier 802. As a consequence, on-axis sound willexperience a gain and the microphone assembly will have a highsignal-to-noise ratio. It is envisioned that a gain of approximately 6dB can be achieved by bi-directional microphone assembly 208. Themicrophone is highly directional, such that off-axis sound isattenuated, and even nulled, by the microphone. Further, thebi-directional microphone assembly 208 can employ any type ofdirectional transducer, so long as identical transducers are employed.

[0088] The bi-directional microphone assembly 208 is schematicallyillustrated in FIG. 9, and alternate embodiments are schematicallyillustrated in FIGS. 10 and 11. As described above, the bi-directionalmicrophone assembly 208 includes transducer 306, having its front faceopening ported to the front ports 312 through channel 506 and its backface openings ported to the back ports 314 through channels 370, 371 and510, and transducer 304, having its front face ported to the rear ports314 through channel 508 and its rear face ported to the front port 312through channels 372, 373 and 512. The bi-directional microphoneassembly 208 thus has transducers mounted on the same lateral axis, butat opposite phases. An alternative to the bi-directional microphoneassembly 208, is the hyper cardioid microphone assembly 1000 illustratedin FIG. 10. The hyper cardioid microphone assembly 1000 includes a fronttransducer 1002 having its front face acoustically coupled to port 1004through channel 1005 and its back face acoustically coupled to port 1006through channel 1009. The front face of a rear transducer 1008 isacoustically coupled to ports 1010 through channel 1011 and the rearface of transducer 1008 is acoustically coupled to port 1006 throughchannel 1012. The transducers are electrically coupled to an operationalamplifier in the same manner that the transducers 304 and 306 areelectrically coupled to operational amplifier 802. However, unlikebi-directional microphone assembly 208, for which identical transducersare selected, the transducers 1002 and 1008, and the variable gainbalance circuit 802, are selected and operated such that the fronttransducer 1002 produces a greater sensitivity than the back transducer1008 while maintaining a null of the vibration created signals.

[0089] The microphone assembly 1000 may be advantageous in applicationswherein the noise incident on the microphone assembly is generallyrandom and omni directional, or in an environment where the front lobeof the microphone needs to be larger to accommodate off-axis noisesources. Microphone assembly 1000 will be better suited for use invehicles where the person speaking, such as the driver, is notpositioned in front of the rearview mirror assembly, because thebi-directional microphone 208 may attenuate the speech from the personspeaking. As noted above, the most intense noise in a vehicle originatesfrom the side of the vehicle, which the bi-directional microphoneassembly 208 mounted to the mirror assembly 101 will better reject thanthe hyper cardioid microphone assembly 1000. Another problematicenvironmental condition better resolved by the bi-directional microphoneassembly 208 than the hyper cardioid microphone assembly 1000, is smallroom reverberation effect. Reverberation causes noise, with a wavelengthlong relative to room dimensions, such that it is omni-directional.Microphone assembly 208, having two identical transducers willeffectively null omni-directional components, such that all thereverberating noise will be cancelled. The hyper cardioid microphoneassembly 1000 will not completely cancel such reverberation noise, dueto the differential on-axis sensitivity for the front and reartransducers 1002, 1008.

[0090] Whereas bi-directional microphone assembly 208 requires matchedtransducers such that the noise is cancelled, the hyper cardioidrequires transducers producing different on-axis sensitivity. Inparticular, the transducer sensitivity differential for transducers 1002and 1008 needs to be 5 to 15 dB, and may for example be 10 dB. Thetransducer control and damping values, which should be considered forthe hyper cardioid microphone assembly 1000, will not be important forthe bi-directional polar microphone assembly 208 so long as thetransducers are the same. So long as identical transducers are provided,the out of phase and the omni-directional contents, such as mechanicalvibration, reverberations, sound having a frequency such that it isnon-directional, will null, in microphone assembly 208. The hypercardioid microphone assembly 1000 requires two different sensitivitiesfrom the front and back transducers 1002 and 1008. The transducers mustbe carefully selected to have the desired sensitivity differential.Microphone assembly 1000 preferably uses higher quality transducers forthe front and back transducers 1002, 1008, so that the desiredperformance can be achieved and sustained, than need be used for thebi-directional microphone assembly 208.

[0091] A second order microphone assembly 1100 according to anotheralternate embodiment is disclosed in FIG. 11. The microphone assembly1100 includes transducers 1102 and 1112. The front face of transducer1102 is coupled to a port 1104 through an acoustic channel 1106. Therear face of transducer 1102 is acoustically coupled to port 1110through channel 1108. The front face of rear transducer 1112 is coupledto port 1110 through channel 1114. The rear face of transducer 1112 iscoupled to port 1116 through channel 1118.

[0092] The transducers 1102 and 1112 are electrically coupled to acircuit 1200 (FIG. 12). The sound from the front transducer 1102 isinput to the non-inverting input of an operational amplifier 802. Thesignal from transducer 1112 is input to a time delay 1202 prior to beinginput to the amplifier 802. The time delay circuit 1202 introduces atime delay equal to the time period required for sound to traveldistance D2, which is the distance from the center of the fronttransducer 1102 to the center of the rear transducer 1112. The delayedsignal is input to the inverting input of the operational amplifier 802through potentiometer R8.

[0093] In operation, the signals originating from the front of themicrophone assembly 1100 will reach the rear transducer 1112 a shorttime period after reaching the front transducer 1102. This time delay isequal to the time required for sound to travel from the center of thefront transducer 1102 to the center of the rear transducer 1112. Sincethe signal entering the rear transducer is electronically delayed intime delay circuit 1202 by an amount equal to the time period requiredfor sound to travel distance D2, the rear signal will arrive at theinverting input of the operational amplifier 802 delayed by a timeperiod equal to twice the time required for sound to travel distance D2.Sound originating from the rear, however will reach front transducer1102 delayed by a time period equal to the time required for sound totravel distance D2. Because the signal from the rear transducer 1112signal is delayed electronically, in delay 1202, by a time period equalto the time required for sound to travel distance D2, the signaloriginating from the back sensed by both transducers 1102 and 1112 willbe input to both the non-inverting and inverting inputs of theoperational amplifier 802 at the same time, such that they are cancelledby the amplifier 802. Accordingly, a null is provided for signalsoriginating from the rear of the microphone assembly. It will berecognized that the greater distances D1 and D2 for the second ordermicrophone assembly 1100, the greater the sensitivity of the microphoneassembly. Additionally, for every distance D2, there is a crossoverfrequency above which the difference in phase no longer adds to theoutput, such that the highest upper frequency desired sets the maximumdistance D2. Above the crossover frequency, the microphone will lose itsdirectional properties and suffer frequency response anomalies. It isenvisioned that the maximum distance D2 for the second order microphoneassembly 1100 will be between 0.75 and 1.4 inches, and may be forexample be approximately 1 inch.

[0094] One issue with respect to this implementation, is the phase shiftthat will occur. In particular, the higher the frequency, the greaterthe phase shift that the signal will experience between the fronttransducer and the rear transducer. Low frequency signals willexperience little phase shift, whereas high frequency signals willexperience a large phase shift. Since acoustic sensitivity increaseswith additional phase shift, low frequency sensitivity will be very low.However, because the signals of interest are voice signals, which arerelatively high frequency signals, the signals of interest will not besignificantly affected by this phase shift. Additionally, it isenvisioned that equalization techniques can be used to compensate forthe phase shift and low frequency roll-off in bass sensitivity of themicrophone 1100. The front and back transducers 1102 and 1112 achieve asecond order directional function by their spacing. Additionally, thetwo transducers face the same direction, such that the front face ofboth the front and rear transducers port forwardly and the back of boththe front and rear transducers port rearwardly. The transducers 1102 and1112 are spaced by a distance D2, which is a dimension close to D1 ofthe front transducer 1102, and may also be a dimension close to the D3for the rear transducer 1112. The greatest output from the microphonewill occur responsive to on-axis sound in front of the microphoneassembly 1100, where the arrival delay is doubled as is the signalstrength.

[0095] The vibration null and additional acoustic advantages ofmicrophone 208 can be gained for the microphone assemblies 1000 and 1100by using four transducers, as illustrated in FIG. 11 for microphoneassembly 1100. In particular, optional transducers 1120 and 1130 areprovided in addition to transducers 1102 and 1112. The rear face oftransducer 1120 is coupled to the front port 1122 via channel 1124 andthe front face of transducer 1120 is coupled to port 1128 via channel1126. The front face of rear transducer 1130 is coupled to rear port1134 via channel 1136 and the back of transducer 1130 is coupled to port1128 via channel 1132. The front transducers 1102 and 1120 are connectedto opposite inputs of the operational amplifier without delay so as tocancel omni-directional noise. The rear transducers 1112 and 1130 aresimilarly connected to opposite inputs of the operational amplifier,after being delayed by the time period required for sound to traveldistance D2, so as to cancel omni-directional noise. Using two pairs oftransducers, each pair will achieve a bi-directional pattern and bedevoid of vibration noise. In particular, nulls will occur at 90, 180,270 degrees. The one main lobe of the microphone assembly 1100 is narrowand forwardly directed, being narrower than the bi-directionalmicrophone assembly 208 forward lobe, and having better off-axis noisecancellation.

[0096] An automatic balancing circuit 1300 (FIG. 13) can be used inplace of, or in addition to, the manual balancing potentiometer R8.Automatic balancing circuit includes a controller 1302 coupled toreceive the output of transducer 304 and variable gain amplifier 1304.The controller generates a gain control signal applied to a variablegain amplifier 1304.

[0097] In operation, the controller monitors the signal levels output bythe transducer 304 and the variable gain amplifier 1304, as indicated inblocks 1402 and 1404 of FIG. 14. The controller monitors for thepresence of speech in step 1406. If speech is present, the controllerdoes not adjust the gain of the variable gain amplifier 1304. If speechis not present, the controller determines whether the output of thevariable gain amplifier 1304 is equal to the output of transducer 304,in step 1408. If it is not equal, the gain of variable gain amplifier1304 is adjusted in proportion to the difference between the signallevel at the output of transducer 304 and the signal level at the outputof amplifier 1304, as indicated in step 1410. The output of the variablegain control will thus be equal to the signal level at the output oftransducer 306, thereby providing noise cancellation. Variation in therelative performance of the transducers 304, 306 over time ortemperature can thus be compensated automatically by the automatic gaincontrol circuit 1300.

[0098] The microphone assemblies 1000 and 1100 can be manufactured inthe same manner as the microphone assembly 208, but with differentspatial relations for the transducers. For example, whereas thetransducers 304 and 306 of microphone assembly 208 are positionedlaterally an equal distance from the front and back ports 312, 314, thetransducers 1002 and 1008 are positioned one behind the other betweenthe front and back ports 1004, 1010, and may for example be positionedalong the longitudinal axis of the microphone assembly 1000, throughwhich the cross section of FIG. 15 is taken. In particular, themicrophone assembly 1000 includes an elastomeric transducer mount 1506into which transducers 1002, 1008 are mounted. The front of transducer1002 ports through channel 1005 and the rear of transducer 1008 portsthrough chamber 1510 and channel 1006. The front face of rear transducer1008 ports through channel 1011 and the rear surface ports throughchamber 1510 and channel 1006. A substantially rigid microphone housing1512 encloses the transducer mount 1506, and includes mechanicalconnectors 1504 for connection to the mirror housing 206, as well asbottom, front and rear ports for sound to enter the microphone forpassage to the transducers. The connectors 1504 can be snap connectorsor connectors that rotate into engagement with the mirror housing in thesame manner as connectors 320. The transducer mount 1506 providesacoustic seal with the transducers 1002, 1008, and the circuit board1502.

[0099]FIGS. 16 and 17 show an alternative structure for microphonesubassembly 1600. Microphone subassembly 1600, as illustrated, includesan electronic portion 1641 which includes a first microphone transducer142 and a second microphone transducer 1644 mounted to a printed circuitboard 1645.

[0100] Microphone transducers 1642 and 1644 are preferably mountedfacing one another or facing away from one another with their centralaxes aligned coaxially. By mounting microphones 1642 and 1644 to faceopposite directions, the sensed pressure waves caused by the vibrationsare sensed 180 degrees out of phase from one another. By mounting themicrophone subassembly to the vehicle such that the common central axisof the transducers is generally aligned with the driver's mouth, theassembly effectively cancels the noise produced by mechanical vibrationsof windshield 20 and the rearview mirror assembly of the vehicle whileincreasing the gain of the driver's speech. A microphone processorcircuit adds the outputs from the two transducers to one another therebynulling any vibration-induced noise.

[0101] As shown in FIG. 17, transducers 1642 and 1644 may be mounted ontheir sides and the subassembly may include acoustic ports that are 90degrees relative to the mechanical axes of the transducers. This allowsboth of the natural transducer front ports to face the redirected frontport of the assembly.

[0102] According to another embodiment, the inventive microphoneassembly utilizes two microphone transducers facing in oppositedirections. The output of the rear facing transducer preferentiallyreceives noise signals while the output of the forward facing transducerpreferentially receives voice signals. Via appropriate electronicprocessing the presence of significant voice signals can be determined.During periods when there are no significant voice signals, output canbe reduced with no harm to voice quality.

[0103] If this processing is done on a frequency band basis, noisedominated bands can be removed with no harm to voice quality since thosebands containing significant voice signals will be passed into theoutput with no alteration.

[0104] Microphone transducers 1642 and 1644 are mounted sideways throughholes formed in printed circuit board 1645. Portions of transducers 1642and 1644 extend below the bottom surface of circuit board 1645 andportions also extend above a top surface of printed circuit board 1645.Mounting the transducers in this orientation and position relative tothe circuit board provides several advantages. First, the electricalcontacts on the transducers may be directly soldered to traces on theprinted circuit board. This avoids the need for manually connectingwires to the transducer contacts and subsequently manually connectingthose wires to the circuit board. Thus, the transducers may be mountedto the circuit board using conventional circuit board populatingdevices.

[0105] Another advantage of mounting the transducers such that theyextend above and below the surfaces of the printed circuit board is thatone side of the circuit board may include a conductive layer serving asa ground plane. Such a ground plane may shield the transducers fromelectromagnetic interference (EMI) that may be produced by othercomponents within the rearview mirror assembly or in other componentswithin the vehicle. Such EMI can introduce significant noise into thesignal delivered by the transducers.

[0106] As shown in FIGS. 16 and 17, microphone subassembly 1600 furtherincludes an acoustic cup 1650 having a pair of central recesses 1652 and1654 arranged to accept the portions of microphones 1642 and 1644,respectively, that extend below the bottom surface of printed circuitboard 1645. Microphone subassembly 1600 further includes a plurality ofports 1655 disposed about the peripheral bottom portion of acoustic cup1650.

[0107] Microphone subassembly 1640 further includes a cloth 1658, whichserves as a windscreen and protects the microphones from the externalenvironment. Cloth 1658 is preferably made of a hydrophobic material andis secured to cup 1650 across ports 1665 to keep water from reachingmicrophones 1642 and 1644.

[0108] Microphone subassembly 1600 also includes the outer microphonehousing 1660 formed in the shape of a cup with a plurality of acousticports 1665 disposed about the bottom and sides of the housing. Ports1665 are preferably aligned with ports 1655 of acoustic cup 1650.Housing 1660 preferably includes one or more posts 1666 a-1666 c thataligns and mates with grooves 1656 a-1656 c in acoustic cup 1650 andgrooves 1646 a-1646 c of printed circuit board 1645. The posts andgrooves serve to align ports 1655 and 1665 while also ensuring that themicrophone transducers cannot rotate or change orientation withinhousing 1660. Housing 1660 further includes a plurality of tabs 1662a-1662 c that resiliently engage the peripheral edge of an apertureformed in housing 206 (FIG. 2). Housing 206 would preferably includecorresponding slots for receiving resilient tabs 1662 a-1662 c to ensurethat microphones 1642 and 1644 are optimally aligned relative to thevehicle.

[0109] While the microphone subassembly is shown in FIG. 2 as beingmounted to the bottom of the mirror housing, it should be noted that thepreferred location is actually on the top of the housing. An example ofa rearview mirror assembly having a microphone subassembly 1600 mountedon the top of the housing is shown in FIGS. 18-20. Microphonesubassemblies mounted on a housing receive not only direct sounds fromthe driver, but also sounds reflected off the windshield. When themicrophone subassembly is mounted on the bottom of the housing, there ismore of a time difference between the arrival of the direct sound andthe reflected sound than when the microphone subassembly is mounted onthe top of the housing. When the arrival times are far enough apart, theresulting combination produces a frequency response that has a series offrequencies with no output. The series, when plotted, resembles a comb,and hence is often referred to as the “comb effect.”

[0110] Mounting the microphone subassembly on top of the housing avoidsthe comb effect in the desired pass band. As shown in the side view inFIG. 20, the distance between the windshield and the top of the housingis much smaller than that at the bottom of the mirror housing and thusthe reflected sound adds correctly to the direct sound creating alouder, but otherwise unaffected, version of the direct sound. The endresult being a higher signal-to-noise ratio and better tonal quality.These are very important attributes in hands-free telephony and vocalrecognition in an automotive environment.

[0111] A problem with mounting the microphone subassembly to the top ofthe housing results from the fact that the microphone assembly is closerto the windshield. When the windshield defroster is activated, a sheetof air travels upward along the windshield. Thus, when the microphonesubassembly is placed on top of the housing, it is exposed to moreairflow as the air from the defroster passes between the housing and thewindow past the microphone subassembly. This airflow creates turbulenceas it passes over the microphone subassembly, which creates asignificant amount of white noise. To solve this problem, a deflector1670 extends upward from the rear of housing 1630 so as to smoothlydeflect the airflow from the defroster over and/or beside microphonesubassembly 1600 so that it does not impact the transducers or createany turbulence as it passes over and around the microphone subassembly.Because the airflow primarily would enter the rear of the microphonesubassembly, the deflector may be designed to redirect the air withminimal impact on the frequency response of the microphone subassembly.This is important for high intelligibility in the motor vehicleenvironment. With no direct air impact and the avoidance of turbulencenear the microphone subassembly, mounting the microphone subassembly onthe top of the housing can offer superior resistance toairflow-generated noise.

[0112] As an additional measure, a signal may be transmitted over thevehicle bus or other discrete wire or wireless communication link, whichindicates that the windshield defroster has been activated. This signalcould be received and processed by the microphone processor and used tosubtract an exemplary white noise waveform that corresponds to thatdetected when the windshield defroster is activated. Alternatively, whenthe system determines that the driver is speaking into the microphoneand that the windshield defroster is activated, the system willtemporarily turn down or turn off the defroster, or otherwise produce asynthesized speech signal advising the driver to turn down thedefroster. The voice recognition circuitry within the mirror may also beutilized for purposes of recognizing noise generated by the defrostersuch that the system will be able to either advise the driver to turnthe defroster down or off or to perform that task automatically.

[0113] In addition to recognizing the sound produced by the windshielddefroster, the microphone may also be used to recognize the sources ofvarious other sounds and hence subtract them from the sound receivedwhile the driver is speaking. For example, the microphone may be used todetect low pass response to determine whether the vehicle is moving.Additionally, the microphone may be used to recognize other events, suchas a door closing or whether the air bags have been inflated. Upondetecting that the air bags have been inflated, the telematics rearviewmirror assembly may be programmed to call 911 and to transmit thevehicle location in a distress signal.

[0114]FIG. 21 shows an exploded view of a microphone assembly 1700constructed in accordance with another embodiment of the presentinvention. Microphone assembly 1700 includes a pair of transducers 1702disposed in apertures 1704 at opposite ends of a transducer boot 1706.Transducer boot 1706 includes an inner cavity 1708 by which the frontsurfaces of transducers 1702 are acoustically coupled and to aforward-facing port 1710 in boot 1706. Transducer boot 1706 is mountedin an aperture 1712 of a circuit board 1714. Thus, a portion oftransducer boot 1706 extends below circuit board 1714 while theremaining portion is positioned above circuit board 1714 with port 1710extending out and resting upon the upper surface of circuit board 1714.

[0115] Microphone assembly 1700 further includes a boot cover 1720. Bootcover 1720 includes a forward opening 1722 that extends over theprotruding port 1710 of transducer boot 1706 so as to allow port 1710 toextend and open outside of boot cover 1720. Boot cover 1720 furtherincludes a pair of tapered side walls 1724 that slope farther aparttoward the rear of transducer boot 1720 where a rear opening 1726 isprovided. In this manner, an acoustic port is provided at the rear ofthe microphone assembly, which is acoustically coupled via the taperedside walls 1724 to the rear surfaces of transducers 1702.

[0116] Microphone assembly 1700 further includes a windscreen 1730,which is preferably a hydrophobic and heat-sensitive adhesive-coatedfabric. Windscreen 1730 is adhesively attached to the underside of amicrophone assembly cover 1732 so as to extend across ports 1734provided in cover 1732. Cover 1732 is preferably tightly bonded aboutcircuit board 1714 to provide a water-impervious enclosure fortransducers 1702.

[0117] Microphone cover 1732 is shown in FIG. 21 as having a generallysquare shape. It should be noted, however, that cover 1732 may be arectangle or other shape and the size and shape of apertures 1734 may bechanged so as to adjust the directionality of the microphone. Further,the acoustic resistivity of windscreen 1730 may be varied to also varythe directionality and polarity of the microphone assembly.Specifically, the acoustic resistivity of windscreen 1730 may beincreased to at least about 1 acoustic Ω/cm² and preferably has anacoustic resistivity of at least about 2 acoustic Ω/cm².

[0118] To illustrate the effect of adjusting the acoustic resistivity ofthe windscreen and the size and positioning of the ports in themicrophone housing cover, the polar patterns were plotted for themicrophone assembly with and without the cover and windscreensurrounding the microphone transducers at four different frequencies,which are plotted in FIGS. 22A-22D and in FIGS. 23A-23D. The polarpatterns (FIGS. 22A-22D) were plotted with the cover and windscreen inplace, and then, the cover and windscreen were removed and the polarpatterns were plotted for the same four frequencies, which are shown inFIGS. 23A-23D. Specifically, the polar patterns shown in FIGS. 22A and23A show the microphone characteristics at 250 Hz, the polar patternsshown in FIGS. 22B and 23B were taken at 500 Hz, the polar patternsshown in FIGS. 22C and 23C were taken at 1000 Hz, and the polar patternsshown in FIGS. 22D and 23D were taken at 2000 Hz. As apparent from acomparison of the respective polar patterns, the rear lobe that ispresent when the cover is not provided over the transducers iseffectively eliminated by appropriately configuring the cover andwindscreen.

[0119] While it has been typical in conventional microphones to minimizethe acoustic resistivity of a windscreen by increasing the porosity ofthe windscreen, the microphone assembly of the present inventionadvantageously utilizes a windscreen with a higher acoustic resistivityby decreasing the porosity of windscreen and yet obtaining not onlybetter water-resistant properties, but to also improved the acousticcharacteristics for the microphone assembly. The reduction of the rearlobe of the polar pattern of the microphone assembly is particularadvantageous when the microphone assembly is mounted on a rearviewmirror assembly since significant noise may be introduced from thewindshield defroster where such noise is typically to the rear and sidesof the microphone assembly.

[0120] When the microphone transducers are sealed in separate housingshaving their own cover and windscreens, the cover ports and acousticresistivity of the windscreens may be different for the differenttransducers so as to compensate for any effects experienced by thetransducers as a result of the positioning of the transducers on thevehicle accessory. For example, when one transducer is mounted closer tothe face of the rearview mirror, its polar pattern is different fromthat of a transducer spaced farther from the mirror surface. Thus, byselecting an appropriate cover design and windscreen resistivity, theeffects of the differences resulting from the positioning of thetransducers may be compensated such that the transducers exhibitsubstantially similar polar patterns and other characteristics. Whilethe windscreen has been described above as consisting of a hydrophobicfabric, it will be appreciated that the windscreen may be moldedintegrally across the ports of the microphone assembly cover. Such anarrangement would simplify the manufacturing of the microphone assemblyby requiring less parts and less manufacturing steps. Further, it wouldmore likely provide more effective seal between the windscreen and thecover.

[0121]FIG. 24 shows yet another embodiment of a microphone assembly2000. As illustrated, microphone assembly 2000 is positioned on the topof a rearview mirror assembly housing 1630 in a manner similar to thatshown in FIGS. 18-20. Similar to that embodiment, a deflector 1670 isprovided that extends from the upper rear portion of housing 1630 so asto provide a relatively flat surface 2005 on which the microphoneassembly 2000 may be mounted.

[0122] Microphone assembly 2000 includes two separate microphonehousings. A first microphone housing 2002 is positioned forward of asecond microphone housing 2004 and is positioned closer to the face ofthe rearview mirror assembly and hence closer to the driver of thevehicle. First microphone housing 2002 includes a cover 2012 having aplurality of ports 2008 through which sound may pass. Second microphonehousing 2004 likewise may include a cover 2014 having a plurality ofacoustic ports 2010. Both housings preferably include a windscreensimilar to that discussed above. The configuration of the ports on thecovers and the acoustic resistivity of the windscreens may be differentfor each of housings 2002 and 2004 so as to compensate for any effectscaused by the positioning of the transducers on the rearview mirrorassembly.

[0123] Each of microphone housings 2002 and 2004 preferably include asingle transducer having its front surface facing the driver of thevehicle. As shown in FIG. 25, the central axes of the transducers andcovers 2012 and 2014 may be aligned along a common axis that is at anangle θ relative to a perpendicular bisector to the rearview mirrorsurface. This is to ensure the transducers are coaxially aligned withthe driver's mouth, since the rearview mirror surface would be at moreof an angle to allow viewing through the rear window of the vehicle. Itshould be noted that the transducers need not be aligned coaxially, butmay be skewed with respect to one another.

[0124] As discussed further below, microphone assembly 2000 ispreferably a second order microphone assembly with the centers of thetwo transducers physically separated by between about 0.75 and 1.4inches, and preferably at 1.3 inches. By spacing the transducers 1.3inches apart, the distance between the transducers is approximatelyone-half the wavelength of sound at 5 kHz. Because of the frequencyresponse of components in existing telephone networks, it may bebeneficial to increase the separation distance between the transducersto between 1.7 and 1.9 inches. Because space may be limited on theaccessory surface on which the transducers are mounted, it may not bepossible to physically separate the transducers by such a distance. Toovercome this problem, a mechanical structure 2006 may be disposedbetween the first transducer and the second transducer to increase theacoustic path length between the first and second transducers.Mechanical structure 2006 may have any symmetrical conical structure andis shown in FIG. 25 as having the shape of a pyramid. As apparent fromFIG. 24, any on-axis sound passing by the first housing 2002 towards thesecond microphone housing 2004 must pass up and over mechanicalstructure 2006. On the other hand, any sound coming off-axis from thesides will still be received at the same time by both microphonestructures 2002 and 2004 regardless of the presence of mechanicalstructure 2006. Test results have shown that a pyramid-shaped mechanicalstructure 2006 having a height of 0.35 inch and side dimensions of 0.70inch with a 45-degree incline of the side surfaces toward the peak thatthe acoustic path length may be increased by approximately 0.35 inch.Thus, greater acoustic separation of the two transducers may be obtainedwithout having to physically separate the transducers by a greaterdistance. This enables the structure to be mounted on relatively smallsurfaces.

[0125] It should be noted that an additional common cover for themicrophone assembly 2000 shown in FIGS. 24-26 may be secured over theillustrated structure provided that the common housing is substantiallyacoustically transparent so as to not effect the arrival times of thesound to the two transducers.

[0126] As shown in FIGS. 24 and 26, a surface of deflector 1670 mayinclude a structure designated as 2020 that is hereinafter referred toas a “fine turbulence generator.” Fine turbulence generator 2020 may beimplemented using a fabric or other fine structure so as to create fineturbulence between deflector 1670 and the laminar airflow along thewindshield defroster as it passes over deflector 1670. A preferred fineturbulence deflector may be implemented using the loop portion of ahook-and-loop-type fastener such as the VELCRO® hook-and-loop fastener.Alternatively, the corresponding surface of deflector 1670 may simply beroughened to create similar turbulence.

[0127] While turbulence generally is undesirable due to the noise itproduces, creating very fine turbulence in the manner proposed createsturbulence having frequency components that exceed the audible limits ofhumans while reducing the turbulence of the air passing by deflector1670 that would produce lower frequency components within the audiblelimits of humans. Because of the fine turbulence created along thesurface of deflector 1670, the laminar airflow is deflected by the fineturbulence that is created rather than the deflector itself. Thisreduces the friction of the deflector as seen by the laminar airflow andtherefore reduces the turbulence created by the airflow that wouldotherwise tend to create lower frequency noise within the audiblefrequencies.

[0128]FIG. 27 shows a block diagram of a preferred microphone processingcircuit 2100 to be used with the second order microphone assembly 2000as depicted in FIGS. 24-26. It will be appreciated, however, thatmicrophone processing circuit 2100 may be used with any second ordermicrophone assembly regardless of whether it is incorporated in arearview mirror assembly, in another vehicle accessory, or in any otheraudio application outside of the vehicle environment.

[0129] Circuit 2100 includes a front transducer 2102 and a reartransducer 2104. As discussed above, for a second order microphoneassembly, front and rear transducers are preferably disposed with theirfront surfaces facing the direction of the person speaking. The output2104 a of rear transducer 2104 is coupled to the input 2106 a of a highpass filter 2106. The output of high pass filter 2106 b is coupled to afirst input 2108 a of a summing circuit 2108.

[0130] The output 2102 a of front transducer 2102 is coupled to theinput of 2110 a of an all-pass phase shifter 2110. The output ofall-pass phase shifter 2110 b is coupled to an inverting input 2108 b ofsumming circuit 2108. As discussed further below, phase shifter 2110 isprovided to shift the phase of the signal from front transducer 2102 byan amount equivalent to the phase shift inherent in high-pass filter2106 such that the signals from front and rear transducers 2102 and 2104have their phase shifted by equal amounts prior to application tosumming circuit 2108 where the signal from front transducer 2102 isinverted and summed with the filtered signal from rear transducer 2104(i.e., the signals are effectively subtracted). The output 2108 c ofsumming circuit 2108 is coupled to the input 2112 a of a three-polehigh-pass filter 2112. The output 2112 b of three-pole high-pass filter2112 may be coupled to the input 2114 a of an optional buffer circuit2114. The output 2114 b of buffer circuit 2114 represents the output ofthe inventive microphone processing circuit.

[0131] Microphone processing circuit 2100 as shown in FIG. 27, includesa biasing circuit 2116, which produces a bias voltage V_(B) that isapplied to each of components 2106-2114, as more apparent from theschematic representations of each of those components. Biasing circuit2116 includes a pair of series-connected resistors 2118 and 2120 coupledbetween a supply voltage V_(S) and ground. Resistors 2118 and 2120preferably have a resistance of 10 kΩ. Biasing circuit 2116 furtherincludes a capacitor 2122 coupled between the output of biasing circuit2116 and ground. Capacitor 2122 preferably has a capacitance of 2.2 μf.

[0132] The details of components 2106-2114 are shown schematically inFIGS. 28A-28E, and are discussed in further detail below following adescription of the general circuit operation.

[0133] To understand the performance and advantages of the inventivemicrophone processing circuit 2100, it is first necessary to understandthe operation of a conventional circuit used with second ordermicrophone assemblies. In prior second-order microphone processingcircuits, the output of the front transducer was simply inverted andprovided to a summing circuit where the signal was summed with thesignal directly supplied from the rear transducer. The frequencyresponse of such a processing circuit is shown in FIG. 29A. In FIG. 29A,plot A shows the sensitivity of the second order microphone assembly atvarious frequencies with the sound originating on-axis. Plot B shows themicrophone sensitivity at various frequencies with the sound originating180 degrees from the axes (i.e., from behind the microphone assembly).Plot C shows the microphone sensitivity for various frequencies arrivingat an angle 90 degrees from the central axes of the transducers (i.e.,directly from the side of the microphone assembly). As apparent fromFIG. 29A, such a microphone circuit is very sensitive to higherfrequencies, but is not very sensitive to lower frequencies within theaudible band for those sounds originating on-axis. To compensate for thelow frequency sensitivity, a high-pass filter may be added at the outputof the summing circuit. While such an arrangement serves to provide amore uniform sensitivity across the frequencies in the audible range,the introduction of the filter renders the assembly extremely sensitiveto vibration-induced noise. More specifically, torsional vibration ofthe transducers is amplified using such a configuration.

[0134] To overcome these problems, the inventive microphone processingcircuit utilizes a high-pass filter 2106 between one of the transducersand summing circuit 2108. High-pass filter 2106 could be placed at theoutput of either front transducer 2102 or rear transducer 2104.High-pass filter 2106 preferably has a characteristic cut-off frequencyat about 1 kHz. By filtering the output of one of the transducers toreduce its bass frequency components prior to subtraction from the othertransducer output, the bass of the resultant output is reduced by asmaller amount than it otherwise would in the absence of filter 2106. Asdiscussed above, all-pass phase shifter 2110 is provided in the path ofthe other transducer so as to ensure that the phase of the signals fromfront and rear transducer 2102 and 2104 are shifted by the same amountprior to reaching summing circuit 2108. FIG. 29B illustrates thefrequency response of the system when phase shifter 2110 is notutilized. As apparent from FIG. 29B, there is a steep drop off inresponse at the middle of the audible range, which results from thephase difference of the signals that would otherwise be applied tosumming circuit 2108.

[0135]FIG. 29C shows the frequency response of the inventive microphoneprocessing circuit 2100 having the construction shown generally in FIG.27 and specifically in FIGS. 28A-28E and described further below. Asapparent from FIG. 29C, the sensitivity of the microphone assembly toon-axis sound is relatively uniform across the audible range. Theon-axis sensitivity is referenced in FIG. 29 as plot A. The 180-degreeoff-axis sound sensitivity is designated in FIG. 29C as plot B. Plot Crepresents the microphone assembly sensitivity to sound arrivingoff-axis at 145 degrees while plot D represents sound originating from apoint 90 degrees off-axis. As apparent from a comparison of these plots,the second order microphone assembly of the present invention issignificantly more sensitive to on-axis sound while is clearly lesssensitive to off-axis sound, particularly at lower frequencies. As notedabove, in an automobile environment, most the noise arrives off-axistowards the sides of the microphone assembly. Thus, the above describedsecond order microphone assembly 2000 and circuitry 2100 issignificantly less sensitive to noise originating from those directions.

[0136]FIG. 28A is a schematic diagram showing the preferred constructionfor high-pass filter 2106. High pass filter 2106 includes a firstresistor 2124, preferably having a resistance of 8.2 kΩ, which iscoupled between filter input 2106 a and supply voltage V_(S). Acapacitor 2126, preferably having a capacitance of 0.001 μf, is coupledbetween input 2106 a and ground. High-pass filter 2106 also includes anoperational amplifier 2128, preferably part No. LM2904, having itsnon-inverting input terminal coupled to bias voltage V_(B), and itsinverting input coupled to input terminal 2106 a via series-connectedcapacitor 2130 and resistor 2132. Capacitor 2130 preferably is a 0.01 μfcapacitor while resistor 2132 preferably has a resistance of 10 kΩ.High-pass filter 2106 also preferably includes a feedback resistor 2134coupled between the inverting input and the output of amplifier 2128.Another resistor 2136 is coupled between the output of amplifier 2128and ground. Preferably, resistors 2134 and 2136 both have a resistanceof 10 kΩ. The output of amplifier 2128 serves as the output 2106 b ofhigh-pass filter 2106.

[0137]FIG. 28B shows the preferred construction of all-pass phaseshifter 2110. Phase shifter 2110 includes a first resistor 2138 that iscoupled between input terminal 2110 a and supply voltage V_(S). Resistor2138 preferably has a resistance of 8.2 kΩ. A capacitor 2140, preferablyhaving a capacitance of 0.001 μf, is coupled between input terminal 2110a and ground. A capacitor 2142 and a resistor 2144 are coupled in seriesbetween input terminal 2110 a and an inverting input of an amplifier2146. Capacitor 2142 preferably has a capacitance of 1 μf. A feedbackresistor 2148 is coupled between the inverting input and the output ofamplifier 2146. A resistor 2150 is coupled between the output ofamplifier 2146 and ground. Amplifier 2146 is preferably part No. LM2904.Another resistor 2152 is coupled between the non-inverting input ofamplifier 2146 and biasing circuit 2116. A capacitor 2154 is coupledbetween the non-inverting input of amplifier 2146 and a terminal betweencapacitor 2142 and resistor 2144. Capacitor 2154 preferably has acapacitance of 0.01 μf. Resistors 2144, 2148, 2150, and 2152 allpreferably have resistances of 10 kΩ. The output of amplifier 2146serves as the output 2110 b of phase shifter 2110.

[0138]FIG. 28C shows a preferred construction for summing circuit 2108.Summing circuit 2108 includes an amplifier 2156 having its non-invertinginput coupled to biasing circuit 2116 so as to receive a bias voltageV_(B). Input terminal 2108 a is coupled to the inverting input ofamplifier 2156 via series-connected capacitor 2158 and resistor 2160.Similarly, input terminal 2108 b is coupled to the inverting input ofamplifier 2156 via series-connected capacitor 2162 and resistor 2164.Capacitors 2158 and 2162 preferably have a capacitance of 1 μf. Aresistor 2166 is coupled between the inverting input and the output ofamplifier 2156. A resistor 2168 is preferably coupled between the outputof amplifier 2156 and ground. Resistors 2160, 2164, and 2168 allpreferably have a resistance of 10 kΩ while resistor 2166 has aresistance of 100 kΩ. Amplifier 2156 is preferably part No. LM2904. Theoutput of amplifier 2156 serves as the output 2108 c from summingcircuit 2108.

[0139]FIG. 28D illustrates a preferred construction for three-polehigh-pass filter 2112. Bypass filter 2112 preferably includes anamplifier 2170 and three capacitors 2172, 2174, and 2176 coupled inseries between input 2112 a and the non-inverting input of amplifier2170. Capacitors 2172, 2174, and 2176 preferably have capacitances of0.33 μf. A resistor 2178 is coupled between ground and a terminalbetween capacitors 2172 and 2174, a resistor 2180 is coupled between theinverting input of amplifier 2170 and a terminal between capacitors 2174and 2176, and a resistor 2182 is coupled between the non-inverting inputof amplifier 2170 and bias circuit 2116. A resistor 2184 is coupledbetween the output of amplifier 2170 and ground. The inverting input andoutput of amplifier 2170 are electrically coupled. Resistor 2178preferably has a resistance of 6.8 kΩ, resistor 2180 preferably has aresistance of 1.1 kΩ, resistor 2182 preferably has a resistance of 270kΩ, and resistor 2182 preferably has a resistance of 10 kΩ. Amplifier2170 is preferably part No. LM2904. The output of amplifier 2170 servesas the output 2112 b of filter 2112. Having this construction, thecut-off frequency of this high-pass filter is about 300 Hz. It should benoted that a different cut-off frequency could be utilized in microphoneprocessing circuit 2100.

[0140]FIG. 28E illustrates a preferred construction for buffer circuit2114. Buffer circuit 2114 preferably includes an amplifier 2186 havingits non-inverting input coupled to input terminal 2114 a via a capacitor2188. A resistor 2190 is coupled between the non-inverting input ofamplifier 2186 and bias circuit 2116. The inverting input of amplifier2186 is coupled to ground via series-connected resistor 2192 andcapacitor 2194. A resistor 2196 is coupled between the inverting inputand the output of amplifier 2186. A resistor 2198 is coupled between theoutput of amplifier 2186 and ground. A capacitor 2199 is coupled betweenthe output of amplifier 2186 and the output 2114 b of buffer circuit2114.

[0141] While the specific circuit implementation is described above formicrophone processing circuit 2100, it will be appreciated by thoseskilled in the art that other configurations may be utilized withoutdeparting from the scope of the invention.

[0142]FIG. 30 shows an alternative microphone processing circuit thatutilizes a digital signal processor (DSP).

[0143] As shown in FIG. 30, the microphone assembly may include one ormore transducers 2210. The microphone processing circuit of themicrophone assembly includes a DSP 2220 and may optionally include apre-processing circuit 2215 disposed between an input to DSP 2220 and anoutput of transducer(s) 2210. Alternatively, DSP 2220 could be coupledbetween pre-processing circuit 2215 and transducer(s) 2210. The outputof DSP 2220 may be applied to various devices such as a voicerecognition device, a recording device, or to a transceiver of a radioor cellular telephone.

[0144] DSP 2220 may be any appropriately configured DSP, but ispreferably either of part nos. TMS320VC5X 5409 or 5402 available fromTexas Instruments. The microphone preferably, but not necessarily,includes two or more transducers arranged as disclosed above, while acorresponding pre-processing circuit such as those disclosed above mayalso be used for circuit 2215. By using two transducers with one spacedfarther away from the person speaking, the arrival time of sounds pickedup by the transducers may be used to determine the likely source of thesounds. For example, the transducer closest to the person speaking willdetect a sound originating from that person before the furthesttransducer. Conversely, any sound that is first detected by the furthesttransducer may be identified as noise. Likewise, any sounds arrivingoff-axis and received by both transducers at the same time may also bediscarded as noise.

[0145] Human vocal cords resonate and thereby create a single frequencywith overtones (also known as harmonics). All vocal cord energy istherefore confined to the harmonics of the vocal cord fundamentalfrequency. For a human male, the fundamental frequency is typicallybetween 35 and 120 Hz, and for a female, the fundamental frequency istypically between 85 and 350 Hz. The DSP filter 2220 of the presentinvention identifies the fundamental frequency of the speech signalsreceived by transducer(s) 2210 and use the identified fundamentalfrequency to compute the coefficients for an inverse comb filter thatwill pass only the harmonics of the vocal cords of the person(s) whosespeech signals are received. In contrast to conventional noise filtersthat try to identify the noise, the inventive filter identifies thespeech. The inventive filter may also be used to separate one talkingperson from another as long as both have different fundamentalfrequencies.

[0146]FIG. 31 shows a process diagram for the adaptive filter asimplemented in DSP 2220. As depicted in block 2225, the analog audiosignal from transducer(s) 2210 is converted into a digital audio signal.A fast Fourier transform (FFT) is then performed on the digitized audiosignal as shown in block 2230. An example of an FFT of an audio signalincluding a speech signal and noise is shown in FIG. 32. Using the FFTof the digitized audio signal, the fundamental frequency of the speechsignal is determined as depicted in block 2235. DSP 2220 identifies thefundamental frequency by identifying frequency components in the FFTthat have amplitudes exceeding a predetermined threshold, and thenidentifying the fundamental frequency as the difference in frequency ofthose frequency components having an amplitude above the predeterminedthreshold. As apparent from the examplary FFT shown in FIG. 32, thehighest peaks are separated by an amount equal to the fundamentalfrequency ƒ₀ and appear at frequencies that are at multiples of thefundamental frequency. Those peaks in the FFT correspond to the harmonicfrequency components of a person's speech.

[0147] After the fundamental frequency is determined in block 2235,adaptive filter coefficients are generated (block 2240) and used toconfigure an inverse comb filter (block 2245) that is used to filter thedigitized audio signal supplied by transducer(s) 2210. An example of aninverse comb filter characteristic is shown in FIG. 33 that is suitablefor filtering a signal having the FFT shown in FIG. 32. The filtereddigital signal may then be converted to an analog speech signal asdepicted in block 2250. For a discussion of how an inverse comb filtermay be configured in a DSP, see Digital Signal Processing Primer, by KenSteiglitz, 1996, ISBN 0-8053-1684-1.

[0148] As shown in FIG. 33, the inverse comb filter passes all frequencycomponents above a predetermined frequency, such as 2500 Hz. This may bedesirable because certain higher frequency sounds in human speech suchas “S,” “Sh,” “T,” and “P” sounds, may not be at a harmonic frequency ofthe vocal cords. In a vehicle environment where much of the noise is atlower frequencies, passing all higher frequency components typicallydoes not present a problem. As described further below, DSP 2220 may beconfigured to predict and hence separate such “S,” “Sh,” “T,” and “P”sounds in human speech from noise at those higher frequencies.Filtering, such as spectral subtraction, can be employed in the regionabove the inverted comb filtering frequencies to reduce noise in thisband.

[0149] By continuously monitoring the incoming audio signal for anychanges in the fundamental frequency, DSP 2220 may adjust the filtercoefficients in response to any detected change in the fundamentalfrequency. The manner in which DSP 2220 adjusts filter components may bepre-configured to prevent abrupt changes that may occur when, forexample, another occupant of the vehicle begins speaking. The desiredfrequency response of the person speaking may thus be estimated andmaintained. Consistency in response is an important factor in speechrecognition. This adjustment is made by comparing the relative intensityof the harmonics over the reference time interval. This relationshipwill then be maintained. For example, in the first few utterances, thesecond average harmonic peak value may be 3 dB greater than that of thethird. If this relationship drifts, the original value will be restored.This concept can also be applied to the relative intensity of thesibilance utterances and the vocal cord levels. The resulting speechoutput may not exactly reproduce a person's normal tonality, but it willreproduce a consistent one. Combined with output level, this adjustmentshould help vocal recognition by removing two very important variables.

[0150] It should also be noted that DSP 2220 may configure two or moresuperimposed inverse comb filters each corresponding to the harmonics ofdifferent individuals in the vehicle. The system may also be taught todefault to the fundamental frequency most often, or last, identifiedupon being activated so as to limit any delay caused by the subsequentidentification of the fundamental frequency.

[0151] Blocks 2255 and 2260 of FIG. 31 illustrate an inventive variablegain adjustment that may optionally be implemented in DSP 2220. The gainof the filtered digitized signal may be varied (block 2255) prior toconversion into an analog signal. The amount that the gain is varied isa function of the noise level detected in the digitized audio signalreceived from transducer(s) 2210 corresponding to a polar pattern with anull facing the direction of the driver, preferably a cardioid or supercardioid.

[0152] A second configuration for DSP 2220 is shown in FIG. 34.According to the second configuration, two transducers are used eachhaving a polar pattern corresponding to a super-cardioid. The firsttransducer 2302 is directed on axis towards the person speaking(typically the driver in an automotive environment), while the secondtransducer 2304 is positioned in the opposite direction with a null inthe polar facing the person speaking. In this manner, while firsttransducer 2302 will pick-up the person's speech as well as some noise,second transducer 2304 will not pick-up the person's speech, but willonly pick up noise including much of the same noise picked-up by firsttransducer 2302. Thus, the output signal of second transducer 2304 maybe subtracted from that of first transducer 2302 to remove unwantednoise. Second transducer 2304 may alternatively haven anomni-directional polar pattern.

[0153] The diagram in FIG. 34 shows that the audio signal of firsttransducer 2302 is converted into a digital audio signal (block 2306)and that the audio signal of second transducer 2304 is also convertedinto a digital, audio signal (block 2308). The digitized audio signalsfrom both transducers are processed to detect the presence of speech(block 2310) and are also both compared to one another (block 2312). Inresponse to the comparison of the signals from first and secondtransducers 2302 and 2304, the gain/phase of the signal from transducer2304 is selectively adjusted (block 2314). The gain/phase adjustedsignal from second transducer 2304 is inverted (block 2316) and issummed with the digitized signal from first transducer 2302 (block2318). The resultant summed signal may optionally be converted into ananalog signal (block 2320). Because the summed signal actuallycorresponds to the subtraction of an adjusted audio signal from secondtransducer 2304 from that first transducer, the summed signal shouldrepresent the speech (if present) with any noise removed. When speech isnot present, however, the summed signal should be a null. Speech may bedetected by performing a FFT on the received audio signal and lookingfrom a fundamental frequency in the range of that expected for a human.

[0154] To appropriately adjust the gain/phase of the signal from secondtransducer 2304, the detection of the presence of speech (block 2310)may be used in the determination of the appropriate gain/phaseadjustment to be made. Further, nulls may be detected in the summedsignal (block 2322) for use in adjusting the gain/phase of the signalfrom second transducer 2304.

[0155] As shown in FIG. 34, some phase adjustment (block 2324) may bedesired to introduce a phase delay into the audio signal from firsttransducer 2302 that corresponds to that inherently introduced duringinversion (block 2316) of the audio signal from second transducer 2304.

[0156] The system in FIG. 34 may be configured to adjust the gain of thesignal only when speech is detected to ensure that the gain is notsuddenly boosted during periods between speech and thereby avoidboosting the noise level during those periods. This configurationovercomes the problems typically associated with using automatic gaincontrol in which the gain is automatically increased during periodsbetween speech and thereby unnecessarily amplifying noise.

[0157] It should be noted that both the functions outlined in FIGS. 31and 34 may be combined in whole or in part to achieve varioussignificant improvements in speech processing.

[0158] The present invention also may use the time relationship betweenvocal cord events and sibilance occurrences to identify the spokenphoneme and recreate it correctly. This may add processing delay butsignificantly improves vocal recognition. Knowing when the vocal eventoccurred the system can look for minor differences relative to thepreceding time interval. There are a limited number of possibilities anddue to noise, nature can be recreated more universally than the moreunique vocal cord noises. For example, the system can determine that a“Sh” sound was uttered and recreate a perfect “Sh” sound. Otherutterances include the “S,” “T,” and “P” sounds. These are all simplenoise bursts of well defined nature.

[0159] The environment around separated transducers significantlydisturbs the frequency response and polar of each transducer. Forexample, a transducer located closer to the front surface of a mirror ina rearview mirror assembly will experience a different polar andfrequency response than a transducer located farther back. The inventivesystem can combine acoustic adjustments and adaptive adjustment tocompensate for these errors. The transducer balance may be adjusted onan adaptive band by band basis to minimize the dominant acoustic noisein each band. This assures the greatest noise reduction possible. Suchan adjustment is preferably performed only during the intervals betweenspeech utterances. Any resulting reduction in speech level will becompensated automatically. Noise reduction will be greater than anyspeech level loss. This assures a maximum signal-to-noise ratio.

[0160] According to another aspect of the present invention, reliablecontinuity is provided through a two wire microphone interface thatremovably couples a microphone assembly to an electronic assembly. Themicrophone assembly includes a power source and a two wire microphoneinterface. The microphone interface includes two contacts that providean audio signal to the electronic assembly. A continuous direct currentis provided through the two contacts such that a low impedance path ismaintained between the microphone assembly and the electronic assembly.

[0161]FIG. 35 depicts a simplified electrical schematic of a microphoneassembly (including a prior art microphone interface) 2400 coupled to anelectronic assembly 2402 (e.g., a differential amplifier stage). Asshown in the circuit of FIG. 35, power is provided to the microphone2400 via a power source (VAUDIO). VAUDIO is coupled to a first end of aresistor R5. A second end of resistor R5 is coupled to a contact 2 of aconnector J1. When mated, contact 2 of connector J1 is coupled to acontact 4 of connector J1 and to a first end of a resistor R6. A secondend of resistor R6 is coupled to a first end of a resistor R14. A secondend of resistor R14 is coupled to a contact 3 of connector J1. Contact 3of connector J1 is coupled to a contact 1 of connector J1, which iscoupled to a first end of a resistor R11. A second end of resistor R11is coupled to a common ground of the electronic assembly 2402.

[0162] In brief, VAUDIO provides power to the microphone assembly via aresistor R5. The current through resistors R5 and R6 provides a chargingcurrent to capacitor C4, which serves to provide a filtered microphonepower supply (VMIC). A continuous wetting current (DC) is provided byVAUDIO through resistor R5, contacts 2 and 4 of connector J1, resistorsR6 and R14, contacts 3 and 1 of connector J1 and resistor R11.Transistor Q1, which is coupled to the first end of resistor R6 and thesecond end of resistor R14, represents the load presented by amicrophone preamplifier.

[0163] Turning to FIG. 36, a simplified electrical schematic of amicrophone assembly 2500 (including a microphone interface, according toan embodiment of the present invention) coupled to an electronicassembly 2502 (e.g., a differential amplifier stage) is shown. VAUDIO iscoupled to a first end of a resistor R5. A second end of resistor R5 iscoupled to a first end of a resistor R6. A second end of resistor R6 iscoupled to a contact 2 of a connector J1. When mated, contact 2 ofconnector J1 is coupled to a contact 4 of connector J1 and a first endof a resistor R12. A second end of resistor R12 is coupled to a firstend of a resistor R8. A second end of resistor R8 is coupled to a firstend of a resistor R13. A second end of resistor R13 is coupled to acontact 3 of connector J1, which is coupled to contact 1 of connectorJ1. Contact 1 of connector J1 is coupled to a first end of a resistorR11. A second end of resistor R11 is coupled to a common ground of theelectronic assembly 2502.

[0164] As shown in FIG. 36, while an auxiliary power supply (V1)provides power to the microphone assembly 2500 (or at least a portion ofmicrophone assembly 2500), the wetting current (DC) is supplied by theelectronic assembly 2502 power source VAUDIO. The wetting current (DC)is supplied from VAUDIO through resistors R5 and R6, contacts 2 and 4 ofconnector J1, resistors R12, R8, R13 and resistor R11. The microphoneinterface, according to the present invention, provides a wettingcurrent for more sophisticated microphone assemblies, such as those thatincorporate digital signal processors (DSPs), which receive power froman auxiliary power source. The present invention allows connectors to beused that have non-precious metal contacts, which reduces the cost ofthe interface while at the same time providing a reliable connectionbetween the microphone assembly 2500 and the electronic assembly 2502.The possible selection of values for resistors R5, R6, R8, R11, R12 andR13 can widely vary provided that the gain and bandwidth of themicrophone assembly and any associated amplifiers are not adverselyaffected. If desired, one of resistors R5 or R6 can be replaced with ashort. Also, resistors R11, R12 and R13 can be replaced with shorts, ifdesired. The value for resistors R8 and R5 or R6 are then selected toprovide an appropriate amount of wetting current. For example, if VAUDIOis twelve volts and a one milliampere wetting current is desired; if a 2kΩ resistor is selected for resistor R5 and resistors R6, R11, R12 andR13 are shorts, then a 10 kΩ resistor is selected for resistor R8. Oneof ordinary skill in the art will appreciate that resistors can be moregenerally an impedance (e.g., R8 can be a choke or active circuit). Thecomponent values indicated in FIG. 36 provide generally acceptableperformance for the microphone assembly utilized.

[0165]FIG. 37 depicts yet another embodiment of the present inventionwhere the wetting current is supplied from the auxiliary power supply(V1). The wetting current (DC) is supplied from power supply V1 throughresistors R5 and R12, contacts 4 and 2 of a connector J1, a resistor R8,contacts 1 and 3 of connector J1 and a resistor R11. If desired,resistors R11, R12 and R13 can be replaced with shorts. The value forresistors R5 and R8 are then selected to provide an appropriate amountof wetting current. The embodiment of FIG. 37 is particularly useful,from the view point of the manufacturer of microphone assembly 2600, inthat the only component that a manufacturer of electronic assembly 2602need provide is resistor R8, across contacts 1 and 2 of connector J1.

[0166]FIG. 38 depicts yet another embodiment of the present inventionwherein the input to the electronic assembly 2702, provided frommicrophone assembly 2700, is balanced. The wetting current (DC) issupplied from power supply (V1) through a resistor R15, a resistor R16,contacts 4 and 2 of connector J1, a resistor R8, contacts 1 and 3 ofconnector J1 and a resistor R20. If desired, resistors R16, R17 and R20can be replaced with shorts. The value for resistors R8 and R15 are thenselected to provide an appropriate amount of wetting current. Thewetting current (DC) can be supplied from a voltage supply, a resistor,a constant current source, inductor or other power source connected toone of the microphone assembly leads. Providing that the microphone hasa DC path for it to complete the wetting current circuit, the source ofthe current is immaterial.

[0167] As shown in FIG. 38, the audio is AC coupled from the microphoneassembly output stage to the electronic assembly 2702. The presentinvention can be extended to multiple connectors that may be includedwithin a microphone assembly or an electronic assembly. According to thepresent invention, all connectors have a DC current flowing through themto maintain a wetting circuit. Thus, oxidation of the contacts will notdisadvantageously affect the circuits utilizing embodiments of thepresent invention. Additionally, the DC voltage of the microphone inputcan be used to verify interface continuity for built in test capability.

[0168] The microphone assembly can be incorporated anywhere in theinterior of a vehicle. For example, the microphone assembly can belocated in the interior trim of a vehicle, in an overhead console,within a visor or within a rearview mirror or the housing of anelectronic rear vision display. In a preferred embodiment, themicrophone assembly is incorporated within an automotive rearviewmirror. If desired, the contacts of the connector that couples themicrophone assembly to the electronic assembly can be plated with aprecious metal (e.g., gold or silver) to facilitate improved continuity.

[0169] Thus, it can be seen that an improved microphone assembly forvehicles is disclosed. It is envisioned that the microphone assembly maybe applied to a wide variety of performance applications, in that themicrophone assembly can include a single transducer or multipletransducers. By using multiple transducers, significantly improvedperformance is achieved. Use of one transducer, having a singlediaphragm or multiple diaphragms suitably ported to achieve a desireddirectional pattern, offers a lower cost microphone that can be used inthe same mount and housing as the multiple transducer microphoneassembly, in applications where the higher performance is not required.

[0170] While the invention has been described in detail herein inaccordance with certain embodiments thereof, many modifications andchanges may be effected by those skilled in the art without departingfrom the spirit of the invention. Accordingly, it is our intent to belimited only by the scope of the appending claims and not by way ofdetails and instrumentalities describing the embodiments shown herein.

What is claimed is:
 1. A rearview mirror assembly for a vehicle,comprising: a mirror subassembly including a mirror housing and adaptedto be attached to the vehicle; a mirror disposed in said mirror housing;and a microphone assembly associated with said mirror subassemblycomprising: a microphone housing having at least one acoustic port; atransducer disposed in said microphone housing; and a windscreen sealedacross said acoustic port, said windscreen having hydrophobic propertiesto prevent water from penetrating said microphone housing through saidacoustic port.
 2. The rearview mirror assembly of claim 1, wherein saidmirror is an electrochromic mirror.
 3. The rearview mirror assembly ofclaim 1 and further including a circuit board having a hole sized toreceive at least a portion of said transducer, wherein said transduceris mounted within the hole in the circuit board such that a portion ofsaid transducer extends below a bottom surface of said circuit board. 4.The rearview mirror assembly of claim 1 and further including a secondtransducer disposed in said microphone housing.
 5. The rearview mirrorassembly of claim 1, wherein said transducer includes a front and a rearsurface, wherein said front surface is acoustically coupled to saidacoustic port in said microphone housing.
 6. The rearview mirrorassembly of claim 1, wherein said microphone housing has at least twoacoustic ports and wherein said windscreen is sealed across saidacoustic ports.
 7. The rearview mirror assembly of claim 6, wherein saidtransducer includes a front and a rear surface, wherein said frontsurface is acoustically coupled to a first one of said acoustic ports insaid microphone housing and said rear surface is acoustically coupled toa second one of said acoustic ports.
 8. The rearview mirror assembly ofclaim 1, wherein said transducer includes a transducer housing having atleast one port.
 9. The rearview mirror assembly of claim 8, wherein saidtransducer is positioned in said microphone housing such that said atleast one port of said transducer housing is spaced apart from saidwindscreen.
 10. The rearview mirror assembly of claim 8, wherein: saidat least one acoustic port in said microphone housing includes at leastone front acoustic port and at least one rear acoustic port; saidwindscreen is disposed across said front and rear acoustic ports; saidat least one port in said transducer housing includes at least one frontport and at least one rear port; said at least one front port of saidtransducer housing being acoustically coupled to said at least one frontacoustic port of said microphone housing; and said at least one rearport of said transducer housing being acoustically coupled to said atleast one rear acoustic port of said microphone housing.
 11. Therearview mirror assembly of claim 10, wherein said microphone housingdefines a common cavity to which both said front and rear ports of saidtransducer housing are acoustically coupled.
 12. The rearview mirrorassembly of claim 1, wherein said at least one acoustic port in saidmicrophone housing includes at least one front acoustic port and atleast one rear acoustic port, and wherein said microphone housingdefines a common cavity to which both said front and rear acoustic portsof said microphone housing are acoustically coupled.
 13. The rearviewmirror assembly of claim 1, wherein said windscreen is adhesivelyattached to the underside of said microphone housing so as to extendacross said acoustic port.
 14. The rearview mirror assembly of claim 1and further comprising a circuit board, wherein said transducer ismounted to said circuit board and said microphone housing is tightlybonded about said circuit board to provide a water-impervious enclosurefor at least a portion of said transducer.
 15. The rearview mirrorassembly of claim 1, wherein said windscreen is molded integrally acrosssaid acoustic port of said microphone housing.
 16. The rearview mirrorassembly of claim 1, wherein said windscreen is made of cloth.
 17. Avehicle accessory for mounting to a vehicle, comprising: a microphoneassembly comprising: a microphone housing having at least one acousticport; a transducer disposed in said microphone housing; and a windscreensealed across said acoustic port, said windscreen having hydrophobicproperties to prevent water from penetrating said microphone housingthrough said acoustic port.
 18. The vehicle accessory of claim 17 andfurther comprising: a circuit board having a hole sized to receive atleast a portion of said transducer, wherein said transducer is mountedwithin the hole in the circuit board such that a portion of saidtransducer extends below a bottom surface of said circuit board.
 19. Thevehicle accessory of claim 17 and further including a second transducerdisposed in said microphone housing.
 20. The vehicle accessory of claim17, wherein said vehicle accessory is a rearview mirror assembly andsaid microphone housing is mounted on said vehicle rearview mirrorassembly.
 21. The vehicle accessory of claim 17, wherein said transducerincludes a front and a rear surface, wherein said front surface isacoustically coupled to said acoustic port in said microphone housing.22. The vehicle accessory of claim 17, wherein said microphone housinghas at least two acoustic ports and wherein said windscreen is sealedacross said acoustic ports.
 23. The vehicle accessory of claim 22,wherein said transducer includes a front and a rear surface, whereinsaid front surface is acoustically coupled to a first one of saidacoustic ports in said microphone housing and said rear surface isacoustically coupled to a second one of said acoustic ports.
 24. Thevehicle accessory of claim 17, wherein said windscreen is adhesivelyattached to the underside of said microphone housing so as to extendacross said acoustic port.
 25. The vehicle accessory of claim 17 andfurther comprising a circuit board, wherein said transducer is mountedto said circuit board and said microphone housing is tightly bondedabout said circuit board to provide a water-impervious enclosure for atleast a portion of said transducer.
 26. The vehicle accessory of claim17, wherein said windscreen is molded integrally across said acousticport of said microphone housing.
 27. The vehicle accessory of claim 17,wherein said windscreen is made of cloth.
 28. The vehicle accessory ofclaim 17, wherein said transducer includes a transducer housing havingat least one port.
 29. The vehicle accessory of claim 28, wherein saidtransducer is positioned in said microphone housing such that said atleast one port of said transducer housing is spaced apart from saidwindscreen.
 30. The vehicle accessory of claim 28, wherein: said atleast one acoustic port in said microphone housing includes at least onefront acoustic port and at least one rear acoustic port; said windscreenis disposed across said front and rear acoustic ports; said at least oneport in said transducer housing includes at least one front port and atleast one rear port; said at least one front port of said transducerhousing being acoustically coupled to said at least one front acousticport of said microphone housing; and said at least one rear port of saidtransducer housing being acoustically coupled to said at least one rearacoustic port of said microphone housing.
 31. The vehicle accessory ofclaim 30, wherein said microphone housing defines a common cavity towhich both said front and rear ports of said transducer housing areacoustically coupled.
 32. The vehicle accessory of claim 17, whereinsaid at least one acoustic port in said microphone housing includes atleast one front acoustic port and at least one rear acoustic port, andwherein said microphone housing defines a common cavity to which bothsaid front and rear acoustic ports of said microphone housing areacoustically coupled.
 33. A vehicle accessory for mounting to a vehicle,comprising: a microphone assembly comprising: a microphone housingcomprising at least one front acoustic port and at least one rearacoustic port, and wherein said microphone housing defines a commoncavity to which both said front and rear acoustic ports of saidmicrophone housing are acoustically coupled; a transducer disposed insaid microphone housing, said transducer including a transducer housinghaving at least one port acoustically coupled to at least one of saidacoustic ports; and a windscreen sealed across said acoustic ports, saidwindscreen having hydrophobic properties to prevent water frompenetrating said microphone housing through said acoustic ports, whereinsaid transducer is positioned in said microphone housing such that saidat least one port of said transducer housing is spaced apart from saidwindscreen.
 34. The vehicle accessory of claim 33 and further comprisinga second transducer disposed in said microphone housing.
 35. The vehicleaccessory of claim 33, wherein said vehicle accessory is a rearviewmirror assembly and said microphone housing is mounted on said vehiclerearview mirror assembly.
 36. The vehicle accessory of claim 33, whereinsaid transducer includes a front and a rear surface, wherein said frontsurface is acoustically coupled to said front acoustic port in saidmicrophone housing and said rear surface is acoustically coupled to saidrear acoustic port.
 37. The vehicle accessory of claim 33 and furthercomprising a circuit board, wherein said transducer is mounted to saidcircuit board and said microphone housing is tightly bonded about saidcircuit board to provide a water-impervious enclosure for at least aportion of said transducer.
 38. The vehicle accessory of claim 33,wherein said windscreen is molded integrally across said acoustic portof said microphone housing.
 39. The vehicle accessory of claim 33,wherein said windscreen is made of cloth.